run killable p associates an id to the process p and executes
it. The execution of this process can be stopped by the emission
of its id on the signal kill. If the execution of p terminates
with the value v, then run killable p returns Some v. If If
the execution of p is aborted, the value None is returned.

run changeable change p state associates an id to the process
(p state) and executes it. The behavior process can be changed
by emitting a new process on the signal change. The state
state is transnited to the new process.

run extensible add p state associates an id to the process
(p state) and executes it. New processes sharing the state state
can be executed in parallel to (p state) by send their
definition on the signal add.